Rotary granulators for cutting up plastics and other like materials, reducing such materials to granular form for re-use or other disposition, are well known in the art. One rotary granulator construction that has been widely used over a substantial period of time is described in Morin U.S. Pat. No. 3,419,223. A comprehensive review of a variety of granulators is presented in "Cutting Chamber is Key to Improved Granulator Performance" appearing in the journal Plastics Machinery & Equipment, April 1983, pages 23,24 and 26.
In the Morin granulator and in virtually all of the various granulators described in the PM&E article, there is a rotor, carrying two or more rotor knives, disposed in a cutting chamber having a lower portion bounded by a perforate screen of semi-cylindrical configuration. At least one bed knife projects radially into the cutting chamber to cooperate with the rotor knives in cutting (granulating) plastic waste or other stock. The material to be cut up in the granulator is fed radially into the chamber through the open side above the rotor opposite the screen. As the stock in the cutting chamber is reduced to granular form, the granulate passes through the screen and is discharged from the granulator. A vacuum or pneumatic conveyor is usually employed to remove the granulated material.
A few machines have used a similar rotary cutting arrangement but have provided for introduction of stock axially into the cutting chamber, passing through a support for the knives. An axial feed arrangement of this kind has the advantage that a substantial portion of the cutting chamber may be encompassed by the perforate screen, allowing a greatly increased area for discharge of granulate material from the cutting chamber. A device of this kind is the wood chipper described in Pallmann U.S. Pat. No. 3,549,093.
There are a number of continuing problems associated with rotary granulators employed in the comminution of waste plastics and similar materials. Machines of this general kind, particularly the radial feed machines that have been most prevalent in the industry, as exemplified by the machines of the aforementioned Morin patent and the PM&E article, are rather limited in through-put, in part because the cutting chamber exit area afforded by the perforate screen is limited to about 180.degree. or less. Such machines frequently exhibit a tendency toward bounce-back of material from the cutting chamber, with unnecessary recirculation before the granulate is discharged from the cutting chamber through the perforate screen. This recirculation effect can lead to the production of excess fines, the particles in many instances being reduced far more than necessary. An axial feed arrangement as in the aforementioned Pallmann patent affords a partial solution to these difficulties, but the obstructions at the inlet to the cutting chamber limit the size of the pieces of stock that can be fed into the machine and also tend to limit the through-put.
Jamming of granulators is also a common problem. In previously known machines, a failure of the vacuum or pneumatic conveyor used to convey granulated material away from the machine may lead to a rapid build-up of material into the area of the screen. If this condition continues for an appreciable period of time, the plastic material being granulated may agglomerate in the screen area, with a potential for substantial damage.
Another common jamming problem of conventional granulators is the jamming or wedging of unground plastic parts or pieces between the screen or knives and the rotor shaft or core. This condition produces damage and deformation of the screen, or full stopping of the granulator rotor.
Rotary granulators, as heretofore known, have also suffered from a substantial noise problem. This noise is in part due to intermittent movements of air through the rotating knives of the machine. Yet another difficulty associated with conventional granulator constructions pertains to the lack of a single design suitable for varying orientations to fit the requirements of varying applications in which the granulator may be employed. That is, the conventional granulator is intended for orientation in just one direction, with the rotor and cutting chamber aligned horizontally; a single design has not been suitable for both vertical and horizontal orientations and the many possible intervening orientations.